The present invention relates to a radio frequency linear accelerator control system, and more particularly &o a system for controlling a resonant cavity type radio frequency linear accelerator so as to be power-supplied always at a frequency tuned precisely with the characteristic resonance frequency of the cavity constituting the accelerator.
It is an essential requirement for a resonant cavity type radio frequency linear accelerator that the frequency of the power supplied to the accelerator should coincide with the characteristic resonance frequency of the cavity constituting the accelerator, because a slight discrepancy between the two frequencies causes a severe decrease in the efficiency of the accelerator owing to a high Q-value feature of the resonant cavity. Meanwhile, though the characteristic frequency of a cavity depends sensitively on the cavity dimensions, they vary owing to an inevitable thermal expansion (or contracton) occurring on the cavity during operation.
According to a conventional resonant cavity type radio frequency linear accelerator, to compensate a cavity resonance frequency change caused by thermal expansion, the cavity, which constitutes the accelerator, is generally provided therein with an externally motor-driven inductive tuner. A radio frequency signal picked up by a small pick-up loop inserted in the cavity has its phase compared at a phase detector with that of the radio frequency power being supplied to the cavity. If the resonance frequency characteristic of the cavity (including the inductive tuner) deviates from the frequency of the power being supplied to the cavity, the phase detector outputs a positive or negative signal reflecting the magnitude and direction of the resonance frequency deviation of the cavity. The output from the phase detector operates the motor driving the above inductive tuner so that the tuner makes the resultant resonance frequency of the cavity coincide with the frequency of the power supplied to the cavity. In this manner the resonance frequency of the cavity can be kept at the same frequency as that of the radio frequency power being supplied to the cavity.
However, such a conventional cavity type radio frequency linear accelerator has a disadvantage that, because the resonace frequency compensation is achieved by a mechanical operation of the inductive tuner, it takes a somewhat long time for the tuner to respond to the resonance frequency deviation. This is unfavorable especially when the deviation is large and abrupt. In addition the inductive tuner must be provided with some slidable electrical contact means for making the tuner continue keeping a good and s&able electric contact with the cavity drum during and after being operated. This not only makes the constitution complex, but also increases the manufacturing cost of the apparatus. Further, for a high power accelerator which is expected to have its temperature raised to a very high level resulting in a large thermal expansion of the cavity, one inductive tuner can not cover a desired extent of compensating the resonace frequency deviation of the cavity. In such a case it is necessary to provide a plurality of inductive tuners or a more powerful cooling means to the cavity. Further, in some cases, the inductive tuners themselves must be provided with cooling means. These also make the apparatus more complex and further expensive.